1. Field of the Invention
This invention relates to stainless steel vacuum bottles and a process for producing the same.
2. Description of the Prior Art
Vacuum bottles which are now widely used are usually made of glass and have a double walled construction with a space between the two walls, said space being evacuated and the surfaces of the walls, which face the vacuum, being coated with silver mirror layers to minimize transfer of heat to or from the interior of the vacuum bottle. Such glass vacuum bottles have high heat-insulating properties, but are poor in resistance to mechanical shocks.
Metal vacuum bottles have been also known and described, for example, in U.S. Pat. Nos. 1,551,778, 1,566,221 and 3,331,522. The vacuum bottles made of steel such as electrolytic iron have been provided with coatings of silver or chromium on surfaces facing the vacuum, i.e., the outer surface of the inner bottle and the inner surface of the outer bottle to minimize transfer of heat to or from the interior of the bottle. These metal vacuum bottles have high resistance to mechanical shocks but are inferior to glass vacuum bottles in the heat-insulating properties because steel has low reflectively and high conductivity. Also, steel is poor in resistance to corrosion, and it is therefore necessary to protect the uncoated surfaces of the vacuum bottle, i.e., the inner surface of the inner bottle and the outer surface of the outer bottle. For this end, these metal vacuum bottles have been mounted in a casing made of plastic material and the inner surface of the inner bottle have been coated with vitreous enamel or plated with chromium. Other metal vacuum bottles, such as that described in U.S. Pat. No. 3,331,522, comprise an inner bottle made of aluminium. Aluminum has very low heat emissivity and high heat reflection, thus making it possible to improve the heat insulating properties. Also aluminum has high corrosion resistance, and therefore makes it unnecessary to protect the inner surface of the inner bottle from the corrosion with a vitreous enamel or nickel plating. However, such vacuum bottles have the following disadvantages. Since aluminium has high heat conduction, it is necessary to make a neck with a low heat conducting material such as stainless steel to minimize heat transfer due to heat conduction. Since aluminium is a soft material and is apt to be deformed by mechanical shocks, it is necessary to make the outer bottle with steel such as mild iron to prevent the same from the damage, resulting in a complex manufacturing process. Also, it is necessary to protect the outer bottle from corrosion.
It has been known that stainless steel has high resistance to corrosion and high mechanical strength, and therefore some attempts have been made to produce vacuum bottles with stainless steel. The stainless steel enables to prevent the vacuum bottles from corrosion and damage, but has a serious disadvantage that the vacuum bottles made of stainless steel are considerably inferior to the glass vacuum bottles in heat-insulating properties which are the most important properties of the vacuum bottles. This problem mainly depends on physical properties of stainless steel and the construction of the vacuum bottles. Firstly, stainless steel is low in heat conductivity but high in heat emissivity, and the transfer of heat due to radiation is a dominant factor in the vacuum bottles. It is therefore difficult to manufacture stainless steel vacuum bottles with good heat-insulating properties. Secondary, metals including stainless steel contain some gases such as hydrogen, and the gases contained therein would be liberated under the reduced pressure from the wall surfaces facing the vacuum. Thus, the vacuum in the space between the two walls would become progressively worse with time. Thirdly, a silver mirror reaction does not occur on the surface of stainless steel. Accordingly, the transfer of heat due to radiation cannot be reduced by providing silver mirror layers on the wall surfaces of the vacuum bottle. For this reason, the prior art vacuum bottles made of stainless steel, such as that described in Japanese patent publication No. Sho 57-22571, have been provided with vitreous layers consisting of silicon dioxide on the wall surfaces facing the vacuum, and silver mirror layers being formed thereon. Such stainless steel vacuum bottles have heat-insulating properties sufficient for the practical use, but it is difficult to form uniform vitreous silicon dioxide layers on the surfaces of the stainless steel, resulting in a wide scatter of qualities of the vacuum bottles. In other stainless steel vacuum bottles, such as that described in Japanese patent application laid-open No. Sho 57-75621, nickel plating have been provided on the wall surfaces facing the vacuum, and silver mirror layers have been formed thereon. These stainless steel vacuum bottles also have heat-insulating properties sufficient for the practical use and maintain their heat-insulating properties for a long time. However, such a construction of vacuum bottles makes a process complicated and causes increase of manufacturing cost because nickel must be plated on individual bottle members before assemblying these members into a double walled bottle.
It has also been known that the stainless steel vacuum bottles may be prevented from lowering of the vacuum by use of a non-evaporable getter which sorps gases liberated from the surfaces of stainless steel facing the vacuum. However, the use of getters makes it complicate to manufacture the vacuum bottles. In general, the stainless steel vacuum bottles having a double walled construction with a space between the two walls, have been produced by a process comprising the steps of preparing bottle members made of stainless steel, plating nickel on surfaces of the members, welding these members to form a double-walled bottle, evacuating the space between the two walls, and then sealing the tip tube. When a getter is employed to prevent the vacuum bottle from lowering of the vacuum, the getter must be arranged in the space between the two walls after forming silver mirror layers but just before welding of the bottle members to form the double-walled bottle. Because, the conventionally used getters such as, for example, Zr-Al alloys loses its functions when it is in contact with water or chemicals. Thus, it is necessary to temporarily assemble bottle members with contractible tubes or rubber rings into a double-walled bottle to form silver mirror layers on the surfaces facing the vacuum and, this necessarily requires disassembling of the double-walled bottle into respective members after formation of silver mirror layers to mount a getter on one of the members, for example, an inner surface of the outer barrel. Accordingly, the prior art vacuum bottles made of stainless steel have problems awaiting solution.